This invention relates to a method of creating electronic images of a finger or other object with ridges and the like, and a compact optical apparatus to project high contrast image slices of a fingerprint onto sensors.
The prior art for contact image sensors is exemplified by U.S. Pat. No. 5,214,273 (xe2x80x9cthe ""273 patentxe2x80x9d) which issued May 25, 1993 for an invention called xe2x80x9cContact Image Sensor.xe2x80x9d A second example of prior art is U.S. Pat. No. 5,331,146 (xe2x80x9cthe ""146 patentxe2x80x9d) which issued Jul. 19, 1994 for an invention called xe2x80x9cContact-type Image Sensor for Generating Electric Signals Corresponding to an Image Formed on a Document.xe2x80x9d Unlike the present invention, the ""273 patent and the ""146 patent do not employ frustrated total internal reflection (FTIR) to view a high contrast fingerprint.
The prior art for fingerprint sensors is exemplified by U.S. Pat. No. 4,784,484 (xe2x80x9cthe ""484 patentxe2x80x9d) which issued November 15, 1988 for an invention called xe2x80x9cMethod and Apparatus for Automatic Scanning of Fingerprints.xe2x80x9d Unlike the present invention, the ""484 patent uses a separate sensing means to measure the speed of finger motion. Also, unlike the present invention, the ""484 patent does not teach the use of FTIR to view a high contrast image of the fingerprint. Finally, unlike the present invention, the ""484 patent does not employ a gradient index rod lens array or an array of relay lens pairs.
The prior art for fingerprint sensors is also exemplified by U.S. Pat. No. 5,619,586 (xe2x80x9cthe ""586 patentxe2x80x9d) which issued Apr. 8, 1997 for an invention called xe2x80x9cMethod and Apparatus for Producing a Directly Viewable Image of a Fingerprint.xe2x80x9d The ""586 patent shows prior art employing FTIR to obtain a high contrast fingerprint image. However, unlike the present invention, the ""586 patent shows imaging of the entire fingerprint at once as an area image, instead of a narrow strip image projected onto a linear array sensor. Also, unlike the present invention, the ""586 patent does not show the use of gradient index rod lenses or relay lenses to image the fingerprint.
The prior art for fingerprint sensors is also exemplified by U.S. Pat. No. 5,096,290 (xe2x80x9cthe ""290 patentxe2x80x9d) which issued Aug. 27, 1990 for an xe2x80x9cApparatus for Imaging Fingerprint Using Transparent Optical Means Having Elastic Material Layer.xe2x80x9d Unlike the present invention, the ""290 patent is covers an elastic layer for the entire area of the fingerprint, rather than a narrow strip of the fingerprint.
The prior art of fingerprint sensors is also exemplified by U.S. Pat. No. 5,448,649 (the ""649 patentxe2x80x9d), which issued September 5, 1995, for an xe2x80x9cApparatus for Imaging Fingerprint or Topographical Relief Pattern on the Surface of an Objectxe2x80x9d. Unlike the present invention, the ""649 patent fails to disclose a stationary lens array and sensor array, does not include a platen with complex geometry and incorporates a physical separation between the platen and the lens array.
The prior art of fingerprint sensors is yet further exemplified by an article entitled xe2x80x9cFingerprint Input Based on Scattered-light Detectionxe2x80x9d, in Applied Optics, Dec. 10, 1997, Opt. Soc. of America, 36(35), 9152-9156. Unlike the present invention, this article fails to disclose the use of stationary lens and sensors, fails to use the light source to illuminate the entire finger, and incorporates a physical separation between the platen and the lens array.
The invention provides a fingerprint image optical input apparatus in the form of a contact image sensor (CIS) which projects high contrast image slices onto a linear array sensor. Novel optics are employed to provide a high contrast image by means of FTIR or direct illumination of ridges of the fingerprint which is projected by a GRIN (GRadient INdex of refraction) lens array onto a linear array sensor.
The generally preferred embodiment is a miniaturized CIS sensor arranged to view the width of the moving fingerprint as it is wiped over the optically transparent platen of the sensor. To view the fingerprint, light must be introduced inside the platen. Light from the light source may be introduced inside the transparent platen through a flat or curved surface, which acts as a lens to direct the light and help collimate the light so that it forms a flat sheet of light the width of the linear array sensor. Once introduced, the light may optionally be directed inside the platen by using total internal reflection (TIR) or reflections from a mirror-like surface or surfaces. The use of reflections to direct the light inside the platen allows the light source to be located anywhere convenient, such as on a printed circuit board; the location of the light source can alter the form of the platen, or allow the platen to be made more compactly. The reflective surfaces or TIR surfaces of the platen can be slightly rough, not perfectly flat, to partially diffuse the light beam and thus cause more even lighting of the fingerprint.
A high contrast image may be obtained by viewing the fingerprint through the transparent platen at an oblique angle; the fingerprint image is then focused by a GRIN lens array onto a linear sensor array. Alternatively, the GRIN lens or other focusing means can be arranged to view reflected images of the fingerprint, and to project reflected images onto the linear array sensor. To provide a high contrast image of the fingerprint, light is directed at an angle to the top interior surface of the platen (typically 45 degrees or more to a line normal to the surface of the platen, depending on the index of refraction of the platen, which is advantageously greater than 1.5), where it is reflected by TIR if no fingerprint is present. Where the fingerprint ridges touch the top surface of the platen, light is not reflected, due to FTIR at the surface of the platen causing absorption of light, resulting in a dark pattern for the fingerprint ridges and bright light at the fingerprint valleys, which are reflected by TIR from the interior of the platen. Foreshortening effects can be accommodated by image processing.
A high contrast image may also be obtained by viewing the fingerprint through the transparent platen at an oblique angle, but with direct illumination of the finger by light directed substantially normal to the internal imaging surface of the platen. The fingerprint image is then focused by a GRIN lens array onto a linear sensor array. Alternatively, the GRIN lens or other focusing means can be arranged to view reflected images of the fingerprint, and to project reflected images onto the linear array sensor. When no finger is present, the light escapes through the surface of the platen, while the GRIN lens sees a black surface by TIR from the platen. A high contrast image of the fingerprint is obtained by when a finger is placed on the platen and the fingerprint ridges are selectively illuminated due to physical contact with the surface of the platen. Where the fingerprint ridges touch the top surface of the platen, the fingerprint ridges glow with scattered light, resulting in a bright pattern for the fingerprint ridges and darker regions at the fingerprint valleys. The oblique viewing angle of the GRIN lens enhances contrast, while allowing limited viewing of fingerprint ridge details that are near to, but not touching, the platen.
The angled surface of the platen can be a raised strip to increase the pressure of the finger on the imaging surface, thereby giving better contact for total internal reflection. The platen surface or raised portion can also be constructed of silicone or some other material with optical wetting or low friction properties to improve imaging or finger movement respectively. A liquid reservoir can be integrated into the platen or an adjacent surface to allowing wetting of the finger with oil or other liquid to improve the total FTIR or fingerprint ridge glow imaging, as well as lubricating the finger for smooth motion. The platen itself can be part of the protective housing of the sensing elements. Surfaces of the platen can be coated in a light absorbing material to absorb stray light, thus reducing the noise at the sensor element.
The high contrast fingerprint image is viewed by a GRIN lens array, or alternatively a relay lens pair array, or any other functionally equivalent means that creates a series of coherent overlapping images. The GRIN lens array looks at an oblique angle to the platen; the narrow fingerprint strip image is focused by the GRIN lens array onto the width of a linear array sensor, which may have one linear array of light sensing pixels or two or more parallel linear arrays of light sensing pixels. The advantages of this arrangement are that a very compact optical system can be achieved which provides fingerprint images which have low distortion, high resolution and large format size.
The CIS sensor for fingerprint imaging can be arranged in several novel configurations, which can be optimized for different applications and manufacturing techniques. For those skilled in the art of design of optical components, the different features of various configurations may be utilized or combined, and other materials, components or technologies may used or combined to achieve substantially similar fingerprint imaging systems.